1. Field of the Invention
The present invention relates to a motorcycle engine improving an arrangement of rotation shafts in an engine case and also relates to a method of assembling the engine case.
2. Related Art
Japanese Unexamined Patent Application Publication No. 2004-360773 discloses a motorcycle engine including an engine case split into two sections, namely, an upper case section and a lower case section. In order to reduce the size of the engine case in the front-rear direction, i.e., longitudinal direction of a motorcycle, a crankshaft and a transmission input shaft (counter shaft) extending in the width direction of the motorcycle are rotatably supported at a mating surface between the upper case section and the lower case section, and a transmission output shaft (driving shaft) rotated by the transmission input shaft is supported in the lower case section.
In this layout of the various rotation shafts, the transmission output shaft is mounted in the engine case through a transmission-output-shaft insertion opening formed in a side wall of the lower case section. The transmission-output-shaft insertion opening is closed in a fluid-tight manner by being covered with a bearing holder holding a bearing of the transmission output shaft. It is required for the transmission-output-shaft insertion opening to have a size such that a transmission driven gear having the largest outer diameter, of multiple transmission driven gears attached to the transmission output shaft, can pass therethrough.
However, when the size of the transmission-output-shaft insertion opening is set to be larger than that of the transmission driven gear having the largest outer diameter, the bearing holder closing the transmission-output-shaft insertion opening inevitably increases in size, and the bearing of the transmission output shaft fitted in the bearing holder is disposed largely apart from a rim of the transmission-output-shaft insertion opening. For this reason, the distance between the bearing of the transmission input shaft and the bearing of the transmission output shaft, that is, the center distance between the axis of the transmission input shaft and the axis of the transmission output shaft is increased. As a result, the size of the engine case is increased.
Further, since the bearing of the transmission output shaft is fixed to the lower case section with the bearing holder disposed therebetween, there is a risk that the positioning accuracy of the bearing will decrease depending on the working accuracy of the transmission-output-shaft insertion opening and the bearing holder.
In addition, after the transmission output shaft having the transmission driven gears is inserted in the engine case through the transmission-output-shaft insertion opening, the bearing holder is mounted. This procedure frequently makes the assembling working difficult.